The Virology Core will serve as a centralized source for well characterized stocks of strains of HIV-1, HIV-2, and SIV. In addition to the standard, prototypic laboratory strains, the Specimen Banking Core will provide isolates from well-characterized clinical material and the Molecular Biology Core will provide genetically defined isolates obtained by DNA transfections of infectious molecular clones. Stocks will be produced in the appropriate host cell (primary CD4+T cells, monocyte- derived macrophages, or cell lines), aliquotted, and archive at -80 C or in liquid nitrogen. The Core will standardize the characterization of theses viruses (which should enhance the quality of the participating CFAR research units) and thereby eliminate much duplication of effort. Three areas of characterization will be performed: (1) tittering of viral strains in the relevant host cells type; (2) phenotyping of viral strains (cell tropism, syncytium-inducing (SI) versus non-SI, cytopathic versus non-cytopathic, replication kinetics, drug susceptibilities, etc.); an (3) studying the effects of investigational interventions (gene therapies, antiviral compounds, antibody neutralization, etc.). In addition, the Core will supply isolated CD4+ cells and macrophages, which are technically difficult to prepare. The availability of these Core viral stocks and services should extend the experimental capabilities of the investigators who use its services.